Method and apparatus for optical stabilization

ABSTRACT

A method and apparatus for controlling the attitude of a vehicle (such as an aircraft) in a space having at least two opposed viewable regions, each region being viewed by a respective first sensor for sensing a first frequency band of electromagnetic radiation and a respective second sensor for sensing a second frequency band of electromagnetic radiation, wherein respective first and second data sets from the first and second sensors for each regions are produced, these second data sets are subsequently modified and combined with the first data sets to form respective third data sets for each region. The attitude of the vehicle is then adjusted until the third data sets are substantially equal.

TECHNICAL FIELD

This invention relates to controlling the attitude of a vehicle,particularly an aircraft, using external sensors such as electromagneticradiation sensors.

BACKGROUND OF THE INVENTION

Controlling the attitude of a vehicle is an important component of anyautopilot or similar control system that acts both to sense and maintainthe orientation of a vehicle according to user-defined conditions.Typically an Inertial Measurement Unit (IMU) is employed as a keycomponent of any such control system. These devices rely on measurementsof changes in the inertial characteristics of a vehicle to determine thevehicle's orientation. However, IMU's suffer from a number ofdisadvantages. An IMU typically requires a number of individual sensorsto be operating, some of which can include rapidly moving parts, inorder to provide sensible information on vehicle attitude. Each of thesesensors requires its own detailed calibration and tuning and as aconsequence IMU's are relatively expensive devices to purchase andmaintain.

Another inherent disadvantage of IMU type systems occurs when a vehicleis subjected to violent turbulence and therefore is rapidly manoeuvring.In these conditions it becomes increasingly difficult to distinguish thedirection of the gravity vector and hence direction from externallyimposed accelerations including centripetal accelerations. These typesof conditions become more pronounced as the size of the vehicle reducesand hence aircraft such as Unmanned Aerial Vehicles (UAV) require moresophisticated and ruggedised IMU's to compensate for these effects. Aswould be expected these higher performance IMU's add a significant costpenalty.

One attempt to provide a system that corrects attitude for an UAV whichdoes not require an IMU is described in U.S. Pat. No. 5,168,152 entitled“Attitude Control with Sensors of a Mining Vehicle”. This documentdescribes a control system for a vehicle which includes Ultra-Violet(UV) light sensors positioned on the vehicle. These sensors areconnected together to produce a signal which is a function of theattitude of the vehicle. This signal is then compared with a commandattitude signal to produce a difference signal which is a measure of thedifference between the attitude of the vehicle and the attitudedetermined by the command attitude signal.

The principle behind this and similar attitude control systems can bereadily explained by reference to FIGS. 1( a) and (b) which depict topand front views of an aircraft 10 including a left looking radiationsensor 1 and its associated field of view and a forward lookingradiation sensor 2 and its respective field of view. Similarly, there isshown a right looking sensor 3 and an aft looking sensor 4. In thevertical plane the radiation sensors are arranged to look outwards withthe centre of the field of view aligned with the horizon as shown inFIG. 1( a). FIG. 1( b) shows the alignment and representative size ofthe lateral fields of view 6, 7 corresponding to the right lookingsensor 3 and left looking sensor 1 respectively.

Referring now to FIGS. 2( a), (b) and (c), aircraft 10 includingradiation sensors as shown in FIGS. 1( a) and (b) is depicted flyingtowards the observer out of the page. Lateral fields of view 6 and 7 arealso shown. Also to the left and right of aircraft 10 arerepresentations of the expected positions of the horizon which directlyrelate to measured radiation sensor intensity. FIG. 2( a) illustratesleft 30 and right 20 lateral views as would be expected as aircraft 10rolls left. Similarly FIG. 2( b) indicates the left 31 and right 21lateral views when aircraft 10 rolls right. Clearly the lateral sensorin the direction of roll will sense more “dark” ground and hence have alower output. When aircraft 10 is not experiencing any roll as shown inFIG. 2( c) the left 32 and right 22 views are substantially equivalentresulting in equivalent intensities being measured by each sensor.

Thus output from the lateral radiation sensors may be used to give anindication of roll and furthermore appropriate control signals can beapplied to controlling elements of the aircraft to cause it to orientitself so as to equalise the light intensities on either side of theaircraft 10. The same principle may be applied to the pitch axis, byincorporating measurements from longitudinal fore 2 and 4 aft radiationsensors. By balancing the measured output from these sensors the pitchof aircraft 10 can be reduced to level. Clearly the combination oflateral and longitudinal looking radiation sensors would allow fullstabilization of attitude in pitch and roll.

As described in U.S. Pat. No. 5,168,152 typically UV sensors are used.This is due to the increased contrast in intensity between the sky andthe ground at wavelengths ranging from blue (450 nm) to UV. This can becompared to measurements in the green to red part of the electromagneticspectrum, where radiation emanated from these regions is of approximateequal intensity. Interestingly, for wavelengths in the near infrared(800 nm) and extending into the millimetre wave band, where the groundappears warm and intense and the sky appears cold and dark, similarcontrasts in measured intensity may be measured.

The approach of using UV or single band radiation sensors suffers from aserious disadvantage which can be readily appreciated by reference toFIG. 3 which depicts aircraft 10 and representations of the lateralviews of radiation sensors similar to that depicted in FIG. 2. In thisexample left lateral view 30 includes the sun, which is a significantsource of UV radiation. This increases the overall intensity measured bythe left viewing sensor of this region of sky with the net effect thatin order to balance this, the aircraft will adjust roll so that rightradiation sensor will measure an equal intensity thus causing theaircraft to roll left. Effectively, the sun is applying a bias to anyroll calculation. This is shown schematically in FIG. 3 where the rollcommand function 50 is directly related to the difference 40 betweenleft and right sensors having left 7 and right 6 lateral fields of view.Given the approximately 2° angular sub tense of the sun, roll errors ofup to 60° are expected for systems employing measurements in the LTVwavelength range. Obviously, the exact same problem will occur in thepitch axis for fore and aft radiation sensors.

One attempt to address this significant problem of attitude bias relieson the fact that the difference in brightness between the sun and thesurrounding sky is dependent on wavelength and ranges from a factor of10 in millimetre wave bands, 100 in thermal bands, to 1000 in visiblebands. Thus the effect of attitude bias introduced by the sun can bemitigated somewhat to approximately 200 by employing thermal bandsensors or down to 6.5° for millimetre wave measurements. However, atdifferent wavelengths cloud cover will have a more pronounced effectalso appearing as an electromagnetic source and resulting in anotherpotential source of error causing an attitude biasing effect similar tothat of the sun.

Another means to attempt to address attitude bias is to narrow the fieldof view of the radiation sensor to reduce the effect. However, thisleads to an attitude control system having less capability to recoverfrom unusual attitudes, and also more prone to biases due to localhorizon deformations (such as trees), and obviously catastrophic failureif the sun should fall within the field of view of any of the radiationsensors.

It is therefore an object of the invention to provide a method andapparatus which improves the performance of attitude stabilisationsystems which employ radiation sensors.

SUMMARY OF THE INVENTION

In a first aspect the present invention accordingly provides a methodfor controlling an attitude of a vehicle in a space having at least twoopposed viewable regions about said vehicle, each region being viewed bya respective first sensor for sensing a first frequency band ofelectromagnetic radiation and a respective second sensor for sensing asecond frequency band of electromagnetic radiation, said methodincluding the steps of:

-   -   a) producing a first data set from said first sensor viewing a        first of said regions;    -   b) producing a second data set from said second sensor viewing        said first region;    -   c) modifying said second data set;    -   d) combining the result of said modifying step with said first        data set to form a third data set for said first region;    -   e) repeating steps a) to d) for a second set of first and second        sensors viewing an opposed viewable region; and    -   f) adjusting the attitude of said vehicle until respective said        third data sets for each opposed viewable region are        substantially equal;

Preferably said steps of modifying and combining reduce a biasintroduced by a source of electromagnetic radiation in a viewableregion. Clearly, this addresses an important disadvantage of prior artmethods wherein introduced attitude biases are not effectivelycompensated for.

Preferably said first frequency band of electromagnetic radiation is inthe ultraviolet frequencies and said second frequency band is in thegreen spectra frequencies and the source of electromagnetic radiation isthe sun. As the sun is one of the most common causes of attitude bias, asystem embodying this method will have improved performance over priorart systems.

In a second aspect the present invention accordingly provides a methodfor calculating the attitude of a vehicle in a space having a viewableregion, said region being viewed by a first and second pair of sensors,each of first and second pair including a first sensor for sensing afirst frequency band of electromagnetic radiation and a second sensorfor sensing a second frequency band of electromagnetic radiation, saidfirst pair of sensors being tilted a first predetermined angle to view afirst sub-region substantially above and including a horizon, and saidsecond pair of sensors being tilted a second predetermined angle to viewa second sub-region substantially below and including the horizon; themethod including the steps of:

-   -   a) producing a first data set from said first sensor of said        first pair;    -   b) producing a second data set from said second sensor of said        first pair;    -   c) modifying said second data set;    -   d) combining the result of said modifying step with said first        data set to form a third data set for said first pair;    -   e) repeating steps a) to d) for said first and second sensors of        said second pair;    -   f) determining a relationship between a change in intensity        between said third data sets and said vehicle attitude; and    -   g) calculating said vehicle attitude from said relationship.

In a third aspect the present invention accordingly provides a methodfor reducing the effects of a source of electromagnetic radiation whenviewing a region to detect variations in background intensity in saidregion, said method including the steps of:

-   -   a) producing a first data set from a first sensor viewing said        region in a first frequency band;    -   b) producing a second data set from a second sensor viewing said        region in said second frequency band;    -   c) modifying said second data set; and    -   d) combining the result of said modifying step with said first        data set to form a    -   third data set for said region, said third data set containing        data wherein said effects of said electromagnetic source are        substantially reduced relative to said variations in background        intensity.

In a fourth aspect the present invention accordingly provides anapparatus for controlling an attitude of a vehicle in a space having atleast two opposed viewable regions about said vehicle, said apparatusincluding for viewing each region, a respective first sensor for sensinga first frequency band of electromagnetic radiation and a respectivesecond sensor for sensing a second frequency band of electromagneticradiation, said apparatus further including:

-   -   a) respective first data capture means for producing first data        sets from said first sensors viewing each of said regions;    -   b) respective second data capture means for producing second        data sets from said second sensors viewing each of said regions;    -   c) respective first processor means for modifying each of said        second data sets;    -   d) respective second processor means for combining the results        of each of said first processor means with said first data sets        to form respective third data sets for each of said regions; and    -   e) control signal generation means for generating a control        signal to adjust the attitude of said vehicle until respective        said third data sets for each opposed viewable region are        substantially equal.

In a fifth aspect the present invention accordingly provides anapparatus for calculating the attitude of a vehicle in a space having aviewable region, said apparatus including for viewing said region, afirst and second pair of sensors, each of first and second pairincluding a first sensor for sensing a first frequency band ofelectromagnetic radiation and a second sensor for sensing a secondfrequency band of electromagnetic radiation, said first pair of sensorsadapted to be tilted a first predetermined angle to view a firstsub-region substantially above and including a horizon, and said secondpair of sensors adapted to be tilted a second predetermined angle toview a second sub-region substantially below and including the horizon;said apparatus further including:

-   -   a) respective first data capture means for producing respective        first data sets from said first sensors of each pair;    -   b) respective second data capture means producing second data        sets from said second sensors of each first pair;    -   c) respective first processor means for modifying each of said        second data sets;    -   d) respective second processor means for combining the results        of each of said first processor means with said first data sets        to form respective third data sets for each pair of sensors;    -   e) third processor means for determining a relationship between        change in intensity between said third data sets and vehicle        attitude; and    -   f) calculating means to calculate said vehicle attitude        according to said relationship.

In a sixth aspect the present invention accordingly provides anapparatus for reducing the effects of a source of electromagneticradiation when viewing a region to detect variations in backgroundintensity in said region, said apparatus including:

-   -   a) first data capture means for producing a first data set from        a first sensor viewing said region in a first frequency band;    -   b) second data capture means for producing a second data set        from a second sensor viewing said region in said second        frequency band;    -   c) first processor means for modifying said second data set;    -   d) second processor means for combining the result of said first        data processor means with said first data set to form a third        data set for said region, said third data set containing data        wherein said effects of said electromagnetic source are        substantially reduced relative to said variations in background        intensity.

In a seventh aspect the present invention accordingly provides a methodfor controlling an attitude of a vehicle said method including the stepsof:

-   -   a) taking a first measurement in a first spectral band;    -   b) taking a second measurement in a second spectral band;    -   c) processing at least one of said first and second measurements        with respect to each other;    -   d) producing a control signal as a result of said processing        step to control said attitude of said vehicle;

In an eighth aspect the present invention accordingly provides anapparatus for controlling an attitude of a vehicle said apparatusincluding:

-   -   a) a first sensor for taking a first measurement in a first        spectral band;    -   b) a second sensor for taking a second measurement in a second        spectral band;    -   c) a processor for processing at least one of said first and        second measurements with respect to each other;    -   d) control signal generator responsive to said processor for        producing a control signal to control said attitude of said        vehicle;

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the followingdrawings in which:

FIGS. 1( a) and (b) show a prior art arrangement of sensors on anaircraft;

FIGS. 2( a), (b) and (c) show a prior art use of the sensors to controlthe attitude of an aircraft;

FIG. 3 shows the situation in which the sun is present in the field ofview of the sensors and the introduced attitude bias;

FIG. 4 shows the use of a method according to an embodiment of thepresent invention to reduce the effect of the sun in the situation ofFIG. 3;

FIG. 5 illustrates the use of image saturation techniques to reduce theeffect of the sun in sensed images according to an embodiment of thepresent invention;

FIG. 6 shows an anti-correlation detection circuit for controlling theattitude of an aircraft;

FIG. 7 depicts another embodiment of the present invention having asensor arrangement employing more radiation detectors to provide angularinformation;

FIG. 8 depicts a circuit according to another embodiment inventionemploying the sensor arrangement of FIG. 7;

FIG. 9 shows a sensor arrangement on an aircraft with the aft sensorremoved.

FIG. 10 depicts an embodiment of the invention being employed as abackup system to a standard Inertial Navigation System (INS);

FIG. 11 shows an embodiment of the present invention being used incombination with a standard GPS system; and

FIG. 12 shows a combination navigation system consisting of an IMU andan optical stabilisation system according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 4, there is shown aircraft 10 incorporating asensor arrangement according to an embodiment of the invention. Viewingthe left side of aircraft 10 are UV sensor 60 and green wavelengthsensor 61. Similarly viewing the right side of aircraft 10 are UV sensor70 and green wavelength sensor 71. Each of these sensors has similarlateral fields of view 6, 7 to the single band sensors depicted in FIGS.1 to 3. Representative views 80 and 81 correspond to measurements fromsensors 60 and 61 respectively on the left side of aircraft 10.Representative views 90 and 91 correspond to measurements from sensors70 and 71 respectively on the right side. As depicted in FIG. 4, the sunis included in the lateral field of view of sensors 60 and 61 viewingthe left hand side of the aircraft. In prior art systems such as thatillustrated in FIG. 3 this would cause aircraft 10 to roll left.

For both sensors 60 and 61, the sun will be the brightest object intheir fields of view. However, UV sensor 60 is highly sensitive to thecontrast between the ground and sky 80 whilst the image 81 viewed bygreen sensor 61 is essentially insensitive to this difference. If anappropriately weighted proportion (factor K) 62 of green sensor 61output is subtracted 63 from UV sensor 60 output, then the effects ofthe sun can be substantially reduced resulting in image 82 for the leftside. A similar process is effected for the right side of the aircraftresulting in image 92. Processed outputs 63 and 73 corresponding toimages 82 and 92 respectively are processed according to a standard rollcommand system 40, 50.

The value K can be determined in a number of ways. In most situationsthe value of K can be preset to a constant value with adequate results.Alternatively, K may be varied according to feedback provided anothersensor such as a magnetic sensor which would detect any inadvertentheading change caused by unintentional bank angle resulting from anincorrect value of K. Clearly, although in this embodiment weighting bya scalar factor has been envisaged, other modifications to the data setfrom green sensor 61 which when combined with the output from UV sensor60 serve to reduce the effects of the sun are contemplated to be withinthe scope of the invention.

As the effect of the sun has been essentially removed from the rollstabilisation system, the roll bias experienced in prior art systems issubstantially reduced thus providing a far more effective attitudecontrol system. Whilst this embodiment has been targeted at removing theeffects of the sun, other electromagnetic sources which may produce abias in the stabilisation system may also be addressed by using suitablesensors which are sensitive to the electromagnetic source but which arerelatively insensitive to differences to intensity between the groundand sky at the given wavelength range.

FIG. 5 depicts representative images 100, 101, 102 corresponding toimages 80, 81, 82 of FIG. 4 for sensors employing a normal chargecoupled device (CCD) or CMOS camera. For these types of devicesbeneficial use of their saturation characteristics can be employedaccording to the invention. As in most cases the sun will saturate theimaging devices, driving them beyond their linear range, then theeffects of the sun can be removed at the pixel level by subtracting thegreen sensor 61 image from the UV sensor 60 image assuming that sensors60, 61 have substantially the same field of view. As the saturationvalues for both types of sensors are similar, the effect of the sun willbe subtracted from resultant image 102 leaving primarily the sky groundcontrast.

In the embodiments of the invention discussed thus far, imaging sensorshave been employed. However, the invention is equally applicable tonon-imaging radiation sensors which may only a have singlephotosensitive element or alternatively be imaging sensors that arede-focussed to the extent that no image formation occurs. In thisinstance there is only a single value corresponding to a radiationsensor measurement rather than a data set. Clearly, data from an imagingsensor may also be processed to produce a single value, however fulladvantage is not then taken of the ability to manipulate data at thepixel level.

A processing advantage is gained if the radiation sensor logarithmicallycompresses the incoming signal. The advantage of this approach is thateach subtraction is equivalent to a division, and thus a ratio.Referring to FIG. 4, at the first level Σ 63, 73 in the block diagram, aratio between ultraviolet and green light is taken, and at the secondlevel Σ 40 a ratio of left to right ultraviolet against green contrastis taken. The division process, easily performed using logarithmicprocessing, eliminates many environmental light level dependentperformance changes. An example of such a performance change is thatwithout logarithmic processing, on a day with half the green light, andhalf the ultraviolet light, the correction signal would be half as much,which is dearly undesirable in a dynamic control system. In the case ofimaging sensors, when individual pixels of the array are logarithmicallycompressed then the sum of the compressed values divided by the numberof pixels forms the logarithm of the geometrical mean of the values. Thequantity is less sensitive to extreme values than the arithmetic mean,and provides an alternative to a saturating nonlinearity.

Referring now to FIG. 6, there is illustrated an embodiment of thepresent invention which incorporates further features to reduce theeffects of horizon asymmetries which become more pronounced during lowaltitude flight. The compensated left and right signals 82, 92corresponding to the left and right sides of aircraft 10 (see FIG. 4)are expected to be closely anti-correlated. As the aircraft rollstowards one side, one side of the aircraft will become brighter as theother becomes darker. If signals 82, 92 are not closely anti-correlatedthen this is due most likely to variations in the horizon between eachside. Such variations could be caused, in one example, by sensors on oneside of the aircraft viewing the ocean and the other side viewing amountain range. Clearly, this will present a dynamically-varying biasinto the attitude control system.

By introducing anti-correlation detector 110 into the attitude controlcircuit the effect of horizon asymmetries can be suppressed. In thisembodiment a Hassenstien-Reichardt anti-correlation detector is use tomeasure the relative degree of correlation between processed signals 63,73 and the associated time delay before correlation.

Anti-correlation detector 110 includes a high pass filtering circuit foreach side 111, 114 which allows only the changing part of the signalfrom one side into the circuit. Each high pass processed signal is lowpass filtered 112, 115 yielding a blurred and delayed version of thesignal. The low pass filtered signal is multiplied with thecorresponding signal from the opposite side 116,113 which will behighest when there is a time delay between raw signals outputs 73, 63that corresponds to the time constant of the low pass filters 111, 114.Summation of the signals at 117 yields a response signal 120 that willonly be strong when there is a delay between left and right light levelchanges, or when there is no change on one side and a change on theother.

When the signals from the opposed radiation signals are closelyanti-correlated detector 110 responds strongly suppressing 120 the rollcommand 50. The suppression system can allow reduced control inputs tocontinue to take place in order to prevent other fault conditionsarising from unusual horizon configurations or gross illuminationasymmetries.

Embodiments of the present invention discussed thus far do not determinethe absolute values of vehicle attitude, whether these be roll or pitch,since the attitude stabilisation system only requires the balancing ofsignals from opposing sides of the vehicle. In any case, changes insensor signal intensity according to change in attitude will varyaccording to the prevailing environmental conditions thus makingdetermination of absolute values problematical without furtherinformation.

Referring now to FIG. 7, there is shown another embodiment of theinvention which includes the ability to determine absolute angles. Inthis embodiment an additional pair of sensors in each spectral band isdeployed on both sides of aircraft 10. Considering the left side ofaircraft 10, radiation sensor pair A consisting of a UV sensor 60 and agreen sensor 61 are tilted to have a lateral field of view substantiallyabove the horizon but still viewing in part the ground. Sensor pair B istilted to have a lateral field of view substantially below the horizonbut still viewing in part the sky. This ensures that there areoverlapping areas viewed by both sensor pairs. Similarly, on the rightside of aircraft 10, sensor pairs A′ and B′ are deployed. By samplingboth above and below the horizon, the gradient of intensity between skyand ground can be determined. Using this information it is possible tocompute angular motion, as the ratio of spatial gradient to temporalgradient in intensity levels by calculating the mean intensitydifference between A and B (and A′ and B′) divided by the angularseparation between A and B (and A′ to B′). Knowing the gradient betweensky and ground allows the flight computer to determine angular positionfrom the intensity of light measured by A, A′, B or B′. Thus a typicalimplementation would have four radiation detectors on each side of eachaxis to be stabilized. However, in another embodiment the number ofsensors may be reduced to three, with only an additional radiationsensor in the wavelength where a large gradient in measured sensorintensity is expected.

Referring now to FIG. 8, an optical stabilisation system incorporatingtwo UV sensors and a green sensor for each side is shown. Sensor group Aand A′ are essentially as shown in FIG. 6 containing pairs of UV 60, 70and green radiation sensors 61, 71 deployed each side of the aircraftand suppression 120 capability based on anti-correlation detection.Secondary UV detectors 140, 150 are oriented towards the ground at aknown angle and provide additional data. Considering now the left sideof aircraft 10, measured signal from upwards looking UV sensor 60 issubtracted 141 from downwards looking UV sensor 140 signal to calculatethe gradient between the ground and sky. This difference value 144 whendivided by the angular difference between the directions of the UVsensors provides an absolute measure between angle change and intensitychange which is continuously updated. An equivalent value 154 is alsocalculated for the right side of the aircraft. The optical stabilisationsystem shown also incorporates high 170 and low 130 rate control similarto that used in control systems that use IMU wherein 170 makes rapidchanges in response to motion and 130 makes gradual corrections inresponse to sustained difference between left and right light levels.Such arrangements have been found to simplify the process of stabilisingautomatically controlled systems.

This technique would enable the optical stabilization system to performmuch of the role of a rate gyroscope about the axis in question. Byproviding angular velocity information to the flight control system, itbecomes less critical that the horizon position control is provided withabsolute angular position, since large corrections are made based onaccurate angular velocities, while small corrections can be made usinglight balance.

In another embodiment a roll stabilisation system according to thepresent invention is mounted on a gimbal having one degree of freedom inthe roll direction. In this embodiment the roll stabilisation systemacts to hold the gimbal level with the horizon and roll angle ismeasured from gimbal position. In this case the control surfaces of theaircraft are commanded by the angle of the gimbal, rather than theoutput of the roll stabilisation system. The use of such a system wouldalso allow the gradient between sky and ground to be adaptivelydetermined by rotating the entire system of the gimbal periodically.This would allow any offset required for banking to be determinedaccurately and allow for an accurate measure of rate of angular motionin the presence of disturbances thus improving the stabilisationsystem's ability to reject disturbances. The gimbal could also be putinto a standby mode when in level flight using the stabilisation systemdirectly on the aircraft control surfaces as described in previousembodiments of the invention to save power.

Although the present invention has largely been described in terms ofroll stabilisation, as this is typically the most critical axisrequiring stabilisation, dearly the invention can be equally applied tothe correction of pitch angle by incorporating fore and aft sets ofappropriate sensors and adopting the methods and systems discussedherein. In the correct circumstances, yaw can also be similarlycontrolled.

Referring now to FIG. 9, there is shown a full attitude stabilisationsystem suitable for both pitch and roll incorporating only three sets ofsensors. The field of view of fore sensor 8 is increased in thehorizontal direction so that there is significant spatial overlapbetween this field of view and the two fields of view of the lateralsensors 1, 3. Thus the lateral sensors can be used as the point ofreference in determining whether the fore sensor is above or below thehorizon by treating the combined signal of the lateral sensors as avirtual aft sensor for comparison. This system is particularly suited tofixed wing aircraft as aft views tend to be dominated by the fuselage ortail plane of the vehicle.

The invention disclosed herein may be used in a number of differentimplementations. It may be used as a standalone system, in combinationwith other navigational aids or a backup system which only operates onfailure of the main control system. Some of these applications will nowbe described in more detail.

As a standalone application the present invention is particularlysuitable for unstable remotely piloted vehicles such as UAVs. Thesevehicles which are often relatively small can be impossible to keepupright when piloted by a human especially for long periods in thepresence of gusts and at high speed. The addition of an opticalstabilisation system according to the present invention would preventthe aircraft from tipping over in flight and reject most gusts allowinga lower crash rate. It will also be appreciated that the necessarycalculations may be done remotely, with sensor data being telemeteredfrom the vehicle to a ground based processor and then resultant controlsignals transmitted in turn to the vehicle.

FIG. 10 illustrates a navigation and control system 200 incorporatingthe present invention to be used in the event of failure of the primaryInertial Navigation System (INS) 220. Many INS systems contain automaticself test systems which are periodically sequenced to detect faults,they do not however indicate what should happen after detection of amajor fault. In standard operation the INS 220 will provide allnavigation and control functionality. When the failure detection system210 detects an INS 220 problem, outputs from the optical stabilisationsystem 230 are used. The amount of reliance on the optical system 230can be staged according to the severity of the fault in the INS 220.

FIG. 11 illustrates a combined direction and control system 300 havingall the essential features of an IMU incorporating a Global PositioningSystem (GPS) 310 to provide heading and location acting in combinationwith an optical stabilisation system 320 according to the presentinvention which provides the attitude information. Ailerons and elevator321 can be used simply to hold the craft upright and level while rudder311 performs the steering for navigation. In those circumstances where arudder 311 is not incorporated in the aircraft such as in many UAVs,knowledge of the dependence of heading change on bank angle as providedby the optical stabilisation system 320 will allow navigation of theaircraft by the aileron and elevator 321.

FIG. 12 illustrates use of an optical stabilisation system 450 accordingto the present invention in a complete autopilot system 400incorporating a low cost IMU 440. As the stabilisation system 450 isunaffected by aircraft accelerations, a low cost IMU may be used. Theimplementation of these techniques would require the use of Kalmanfilters 430, extended Kalman filters, or similar optimal filters, inorder to adapt in flight to changes in the environment and combinemeasurements from external navigation references with measured changesin light distribution. Using these techniques the performance of the IMU440 and stabilisation system 450 would be higher than that obtained foreither sensor operating individually.

Although a preferred embodiment of the method and apparatus of thepresent invention has been illustrated in the accompanying drawings anddescribed in the foregoing detailed description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the scope of the invention as set forth anddefined by the following claims.

1. A method for reducing effects of a source of electromagneticradiation when viewing a region to detect variations in backgroundintensity in said region, comprising: a) producing a first data set froma first sensor viewing said region in a first frequency band ofelectromagnetic radiation; b) producing a second data set from a secondsensor viewing said region in a second different frequency band ofelectromagnetic radiation; c) modifying said second data set; and d)combining said modified second data set with said first data set,whereby said effects of said electromagnetic source are reduced relativeto said variations in background intensity.
 2. A method as claimed inclaim 1, wherein measurements by said second sensor are more sensitiveto said electromagnetic source than to intensity differences betweenvariations in background intensity in said second frequency band.
 3. Amethod as claimed in claim 1, wherein said modifying includesmultiplying said second data set by a predetermined factor.
 4. A methodas claimed in claim 1, wherein said combining includes finding adifference between said modified second data set and said first dataset.
 5. A method as claimed in claim 1, wherein said producing saidfirst and second data sets further includes logarithmically compressingsaid data sets.
 6. A method as claimed in claim 1, wherein said firstfrequency band is in the ultraviolet, said second frequency band is inthe green spectra, and said source is the sun.
 7. A method as claimed inclaim 1, wherein at least of one said first and second frequency bandsis or is in the millimeter wavelength band.
 8. An apparatus for reducingeffects of a source of electromagnetic radiation when viewing a regionto detect variations in background intensity in said region, comprising:a) a first connection providing a first data set from a first sensorviewing said region in a first frequency band of electromagneticradiation; b) a second connection providing a second data set from asecond sensor viewing said region in a second different frequency bandof electromagnetic radiation; c) a processor coupled to said first andsecond connections and which modifies said second data set and combinessaid modified second data set with said first data set, whereby saideffects of said electromagnetic source are reduced relative to saidvariations in background intensity.
 9. An apparatus as claimed in claim8, wherein measurements by said second sensor in said second frequencyband are more sensitive to said electromagnetic source than to intensitydifferences between variations in background intensity in said secondfrequency band.
 10. An apparatus as claimed in claim 8, wherein saidprocessor multiplies said second data set by a predetermined factor. 11.An apparatus as claimed in claim 8, wherein said processor finds adifference between said modified second data set and said first dataset.
 12. An apparatus as claimed in claim 8, wherein said firstfrequency band is in the ultraviolet, said second frequency band is inthe green spectra, and said source is the sun.
 13. An apparatus asclaimed in claim 8, wherein at least of one said first and secondfrequency bands is or is in the millimeter wavelength band.